AX is a covalent diatomic molecule where A and X are second row elements of periodic table. Based on Molecular orbital theory, the bond order of AX is 2.5. The total number of electrons in AX is ________. (Round off to the Nearest Integer).

We are told that AX is a covalent diatomic molecule where both A and X are second-row elements (Li through Ne), and its bond order according to Molecular Orbital Theory is 2.5. We need to find the total number of electrons in AX.

Recall that for common diatomic molecules, the bond order is calculated as $$\text{B.O.} = \frac{N_b - N_a}{2}$$, where $$N_b$$ is the number of bonding electrons and $$N_a$$ is the number of antibonding electrons. A bond order of 2.5 is characteristic of molecules with an odd number of electrons.

The well-known diatomic species with bond order 2.5 among second-row elements is $$\text{NO}$$ (nitric oxide). Nitrogen has 7 electrons and oxygen has 8 electrons, giving a total of $$7 + 8 = 15$$ electrons. The MO configuration of NO is: $$$(\sigma 1s)^2\, (\sigma^* 1s)^2\, (\sigma 2s)^2\, (\sigma^* 2s)^2\, (\sigma 2p_z)^2$$$ $$$(\pi 2p_x)^2\, (\pi 2p_y)^2\, (\pi^* 2p_x)^1$$$, which gives bond order $$= \frac{10 - 5}{2} = 2.5$$.

Therefore the total number of electrons in AX is $$15$$.